Dual-material mandrel for epitaxial crystal growth on silicon

ABSTRACT

In one example, a method for fabricating a semiconductor device includes etching a layer of silicon to form a plurality of fins and growing layers of a semiconductor material directly on sidewalls of the plurality of fins, wherein the semiconductor material and surfaces of the sidewalls have different crystalline properties.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to semiconductor devices andrelates more specifically to multiple gate field effect transistors.

BACKGROUND OF THE DISCLOSURE

Multiple gate field effect transistors (FETs) aremetal-oxide-semiconductor field effect transistors (MOSFETs) thatincorporate more than one gate into a single device. A finFET is aspecific type of multiple gate FET in which the conducting channel iswrapped by a thin fin forming the body of the device. The effectivechannel length of the device in this case is determined by the thicknessof the fin (measured from source to drain). The wrap-around structure ofthe gate provides improved electrical control over the channel, and thushelps mitigate leakage current and other short-channel effects.

SUMMARY OF THE DISCLOSURE

In one example, a method for fabricating a semiconductor device includesetching a layer of silicon to form a plurality of fins and growinglayers of a semiconductor material directly on sidewalls of theplurality of fins, wherein the semiconductor material and surfaces ofthe sidewalls have different crystalline properties.

In another example, a method for fabricating a semiconductor deviceincludes depositing a buried oxide layer directly upon a substrate,depositing a silicon-on-insulator layer directly upon the buried oxidelayer, and depositing a hard mask directly upon the silicon-on-insulatorlayer, where the hard mask comprises a first material layer depositeddirectly upon the silicon-on-insulator layer and a second material layerdeposited directly upon the first material layer, and where the firstmaterial layer and the second material layer are formed from differentmaterials. The hard mask is patterned to create a plurality of fins, andthe silicon-on-insulator layer is etched in a manner that removesportions of the silicon-on-insulator layer not residing directly beneaththe plurality of fins. A plurality of spacers is formed along exposedsurfaces of the silicon-on-insulator layer, and a surface oxide isdeposited over the plurality of spacers, where surface oxide fills inspaces between the plurality of fins. The second material layer of thehard mask and the plurality of spacers are removed in a manner that isselective to the first material layer of the hard mask and to thesilicon on insulator layer, and the removal exposes sidewalls of thesilicon-on-insulator layer. A semiconductor material is grown directlyon the sidewalls of the silicon-on-insulator layer, where thesemiconductor material is a different material from the material of thesilicon-on-insulator layer. The surface oxide, the first material layerof the hard mask, and the silicon-on-insulator layer are removed in amanner that is selective to the semiconductor material.

In another example, a semiconductor device includes a plurality of finscomprising silicon. A layer of a semiconductor material is growndirectly on a sidewall of at least one fin of the fins. Thesemiconductor material and the surface of fin have different crystallineproperties.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present disclosure can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIGS. 1A-1H illustrate a semiconductor device during various stages of afirst fabrication process performed according to examples of the presentdisclosure;

FIG. 2 illustrates a top view of a first example of a semiconductordevice fabricated according to the process illustrated in FIGS. 1A-1H;and

FIG. 3 illustrates a top view of a first example of a semiconductordevice fabricated according to the process illustrated in FIGS. 1A-1H.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe Figures.

DETAILED DESCRIPTION

In one example, a dual-material mandrel for epitaxial crystal growth onsilicon is disclosed. Semiconductor materials such as Groups III-Vmaterials have been used to form transistors including finFET devices.These materials are typically difficult to obtain in bulk crystal form,and often must be grown on substrates. However, the differences in thecrystalline properties of the semiconductor film and the substratesurface (e.g., different lattice constants) complicate growth of thesemiconductor materials. Thick buffers deposited between the substratesurface and the semiconductor materials can facilitate growth; however,they also take up space on a device whose dimensions are already verylimited without improving device operation.

Examples of the present disclosure provide a dual-material mandrel forepitaxial crystal growth on silicon that eliminates the need for a thickbuffer at the substrate/semiconductor device interface. In one example,a hard mask comprising two material layers formed from differentmaterials (e.g., an oxide and a nitride) is used to pattern a layer ofcrystalline silicon. A first of the material layers is removed to createtrenches in which a semiconductor material, such as a Group III/V, GroupII/IV, or Group IV semiconductor material can be grown directly onto thesidewalls of the patterned silicon, without the need for a buffer inbetween the silicon and the semiconductor material. The second of thematerial layers constrains the growth of the semiconductor material tothe silicon sidewalls and is removed after the semiconductor materialhas been grown.

FIGS. 1A-1H illustrate a semiconductor device 100 during various stagesof a first fabrication process performed according to examples of thepresent disclosure. As such, when viewed in sequence, 1A-1H also serveas a flow diagram for the first fabrication process. In particular,FIGS. 1A-1H illustrate cross sectional views of the semiconductor device100 during the various stages of the first fabrication process.

Referring to FIG. 1A, one example of the semiconductor device 100 beginsas a wafer or substrate 102, formed, for example, from bulk silicon(Si), a Group III-V material, a Group III-V material on silicon, orother semiconductor materials. In one example, the crystal orientationof the top surface of the substrate 102 (i.e., the surface upon whichthe rest of the structure of the semiconductor device 100 is built) isdefined by a Miller index of (100) or (110). A buried oxide layer 104,formed, for example, by bonding or high energy oxygen implantation, isbonded directly on the wafer 102. A silicon-on-insulator (SOI) layer 106is bonded directly on the buried oxide layer 104. In one example, thesilicon of the SOI layer 106 has a crystalline orientation. In oneexample, the crystal orientation of the top surface of the SOI layer 106is defined by a Miller index that is different from the Miller indexdefining the crystal orientation of the top surface of the substrate102. For instance, if the crystal orientation of the top surface of thesubstrate 102 is defined by a Miller index of (100), the crystalorientation of the top surface of the SOI layer 106 may be defined by aMiller index of (110).

As illustrated in FIG. 1B, a mandrel or hard mask is next depositeddirectly on the SOI layer 106. The hard mask comprises a first materiallayer 108 and a second material layer 110 formed from two differentmaterials. For instance, the first material layer 108 may be formed froman oxide, while the second material layer 110 may be formed from anitride. Next, the first material layer 108 and the second materiallayer 110 of the hard mask are patterned to create a plurality of“fins.” Any number of fins may be created as a result of this patterningprocess, and the spacing between the individual fins is variable.Patterning of the first material layer 108 and the second material layer110 involves etching the first material layer 108 and the secondmaterial layer 110 down to the SOI layer 106, and may further involvethe deposition of additional layers of material, such as additionalmasks or other sacrificial materials (not shown), that are removed inthe process of creating the structure illustrated in FIG. 1B. In oneexample, the patterning of the first material layer 108 and the secondmaterial layer 110 results in fins whose sidewalls are aligned to thecrystal plane of the SOI layer 106 that is defined by a Miler index of(111), although other crystal orientations are also possible.

As illustrated in FIG. 1C, the SOI layer 106 is next etched down to theburied oxide layer 104. In one example, the etching of the SOI layer 106is a wet etch or reactive ion etch (RIE) process. As illustrated, theportions of the SOI layer 106 that reside below the hard mask remainafter the etching, and form the lower portions of the “fins” that werecreated by the patterning of the hard mask in FIG. 1B. In one example,the etching of the SOI layer 106 cuts into the silicon of the SOI suchthat the exposed surface of the SOI layer 106 is aligned in a crystalorientation that is defined by a Miler index of (111).

As illustrated in FIG. 1D, a plurality of spacers 112 ₁-112 _(n)(hereinafter collectively referred to as “spacers 112”) is next formedalong the sidewalls of the fins. The spacers 112 thus contact the SOIlayer 106 and the first material layer 108 and the second material layer110 of the hard mask. In one example, the spacers 112 are formed fromthe same material as the second material layer 110 of the hard mask,e.g., from a nitride.

As illustrated in FIG. 1E, a surface oxide layer 114 is next depositedover the semiconductor device 100. The surface oxide layer 114 directlycontacts the second material layer 110 of the hard mask, the spacers112, and the buried oxide layer 104 and fills in the spaces between thefins. The surface oxide layer 114 is then planarized. As illustrated,planarization of the surface oxide layer 114 may additionally result inthe planarization of portions of the spacers 112 and the second materiallayer 110 of the hard mask.

As illustrated in FIG. 1F, the second material layer 110 of the hardmask and the spacers 112 are next removed. In one example, removal ofthe second material layer 110 of the hard mask and the spacers 112 isperformed using an etch process that is selective to (i.e., does notremove) the portions of the buried oxide layer 104, surface oxide layer114, and the SOI layer 106 residing beneath the second material layer110 of the hard mask and the spacers 112. Thus, the removal of thesecond material layer 110 of the hard mask and the spacers 112 resultsin a plurality of trenches 116 ₁-116 _(n) (hereinafter collectivelyreferred to as “trenches 116”) being formed around the fins, in theareas where the spacers 112 used to be. This exposes the sidewalls ofthe fins (i.e., the sidewalls of the SOI layer 106 and the firstmaterial layer 108 of the hard mask).

As illustrated in FIG. 1G, semiconductor channels 118 ₁-118 _(n)(hereinafter collectively referred to as “semiconductor channels 118”)are next grown epitaxially directly on the sidewalls of the fins, i.e.,in the trenches 116. In one example, growth of the semiconductorchannels 118 is limited to the sidewalls of the SOI layer 106; thus, thesemiconductor channels 118 directly contact the buried oxide layer 104and the SOI layer 106 and do not extend to the first material layer 108of the hard mask. The semiconductor channels 118 are formed from asemiconductor material having different properties from the sidewallsurfaces of the SOI layer 106 (e.g., different lattice constants) butthe same crystal orientation. In one example, the semiconductor channels118 are formed from a Group III/V, Group II/IV, or Group IVsemiconductor, such as indium gallium arsenide (InGaAs).

As illustrated in FIG. 1H, the surface oxide layer 114, the firstmaterial layer 108 of the hard mask, and the SOI 106 are next removed.In one example, the surface oxide layer 114, the second material layer108 of the hard mask, and the SOI 106 are removed using an etch process,such as a xenon fluoride gas etch or a tetramethylammonium hydroxide wetanisotropic etch. As illustrated, the etching is selective to (i.e.,does not remove) the epitaxially grown semiconductor channels 118.

The resultant semiconductor channels 118 may form the conductingchannels of a finFET device. The finFet device may be an N-type device(NFET) or a P-type device (PFET). Thus, Groups III-V semiconductorchannels may be grown directly on a silicon surface having a differentcrystalline structure, without the use of a thick buffer. Thus, devicespace is not wasted on buffers that provide no operational advantage. Inthe case of a PFET, a narrower version of the SOI layer 106 can be usedas is to form the dual material fins.

Thus, the disclosed dual-material mandrel or hard mask eliminates theneed for a thick buffer at the substrate/semiconductor device interfaceduring fabrication of the semiconductor device 100. The only templateneeded to grow the semiconductor channels 118 is the SOI layer 106,which is small and thin relative to the typical buffer (which can beseveral μm thick and wide).

The process illustrated in FIGS. 1A-1H may be used to fabricatesemiconductor channels in a plurality of different patterns upon theburied oxide layer, including one- and two-dimensional patterns. FIG. 2,for example, illustrates a top view of a first example of asemiconductor device 200 fabricated according to the process illustratedin FIGS. 1A-1H. In particular, FIG. 2 illustrates a semiconductor device200 in which the semiconductor channels have been fabricated into aone-dimensional pattern. The buried oxide layer 202 is visible from thetop, as are the semiconductor channels 204 ₁-204 _(m) (hereinaftercollectively referred to as “semiconductor channels 204”), which arearranged in a set of parallel continuous lines. The arrangementillustrated in FIG. 2 is obtained by performing the process illustratedin FIGS. 1A-1H along only one of the x and y dimensions of thesubstrate.

FIG. 3 illustrates a top view of a first example of a semiconductordevice 300 fabricated according to the process illustrated in FIGS.1A-1H. In particular, FIG. 3 illustrates a semiconductor device 300 inwhich the semiconductor channels have been fabricated into atwo-dimensional pattern. The buried oxide layer 302 is visible from thetop, as are the semiconductor channels 304 ₁-304 _(k) (hereinaftercollectively referred to as “semiconductor channels 304”), which arearranged in a rectangular matrix. The arrangement illustrated in FIG. 3is obtained by performing the process illustrated in FIGS. 1A-1H alongboth of the x and y dimensions of the substrate.

Although various embodiments which incorporate the teachings of thepresent invention have been shown and described in detail herein, thoseskilled in the art can readily devise many other varied embodiments thatstill incorporate these teachings.

What is claimed is:
 1. A structure, comprising: a silicon substratehaving a top surface having a first crystal orientation defined by aMiller index of (100); a buried oxide layer having a first side bondeddirectly on the top surface of the silicon substrate; and a plurality ofconducting channels positioned directly above a second side of theburied oxide layer that is opposite to the first side, such that theburied oxide layer separates the silicon substrate from the plurality ofconducting channels, wherein each channel of the plurality of conductingchannels is formed from an epitaxially grown non-silicon semiconductormaterial on sidewalls of a silicon-on-insulator layer and a surfaceoxide layer and thereafter removing the silicon-on-insulator layer andthe surface oxide layer selective to the non-silicon semiconductormaterial, and wherein each channel of the plurality of conductingchannels has sidewalls having a second crystal orientation defined byanother Miller index of (111).
 2. The structure of claim 1, wherein thenon-silicon semiconductor material is a Group III/V material.